The ability of a mobile or fixed wireless transmit/receive unit (WTRU) to acquire beacon channels in a fast and reliable manner is a key functionality for a Third Generation (3G) Time Division Duplex (TDD) system. Beacon channels, also known as broadcast control channels (BCHs), transmit in a predefined slot within a particular frame of a multi-frame TDD system.
A beacon channel is always a BCH, but other channels such as the paging indicator channel (PICH) or a paging channel (PCH) can also be used as beacon channels. However, an important characteristic of a beacon channel is that it must be transmitted at a fixed (high) reference power level such that the beacon can be reliably received everywhere in the cell. This allows a WTRU to determine the pathloss estimate from the known beacon channel. In the favored deployment of a TDD system, an unlimited number of base stations (BSs) may transmit their beacon signals in the same time slot, allowing all the WTRUs in the coverage area to measure all the beacons simultaneously. The WTRUs can then compare the power received from each of the BSs in the coverage area and choose to connect to the BS with the highest quality signal.
The WTRU acquires the beacon for system access information or to find other WTRUs for eventual cell handover or cell re-selection. The advantage of having all the neighboring BS beacons in the same timeslot also leads to a disadvantage in the form of interference from all the high powered signals in the same time slot.
A beacon channel can be further defined as a special physical channel that is sent at a fixed high reference power and uses a special reserved orthogonal variable spreading factor (OVSF) code, which is sent at least once per frame. A time slot (TS) that contains a beacon channel is called a beacon TS.
There are two common deployment scenarios for beacon time slots in TDD systems. In the first scenario, only a single time slot is allocated as the beacon TS of a cell. The single beacon TS containing the BCH is always found at a specific time slot location TS(k) of the frame. In the second scenario, two time slots are allocated as the beacon TSs of a cell. The BCH beacon information is sent at location TS(k) of the frame and the second beacon TS of the frame is located at TS(k+8). The second beam TS is known as the secondary beacon and it may contain other downlink channel information. The second scenario represents the predominant deployment of a TDD system today.
TDD cells which operate in close geographical areas and on the same frequencies need resource coordination and time synchronization in order to achieve maximum system capacity and efficiency. The deployment of the beacon TSs of TDD cells uses a scheme where the beacon channels of all neighboring cells are sent in the same time slot, thus requiring time alignment. The major benefit of time-aligned beacons is that it allows WTRUs to simultaneously measure their neighboring cell BS and the current serving cell BS. The WTRU may discover another BS with a better signal level and switch to that BS, thereby allowing the WTRU to reduce its transmitting power and preserve battery life. However, if the coverage area has many BSs in close proximity, there is a strong possibility that the time-aligned beacon TSs will lead to extremely degraded BCH beacon acquisition performance for WTRUs.
To study the acquisition time of the BCH beacon, a simple geometric arrangement based upon path-loss shows that a WTRU at a cell's border, (equally distant between two neighboring BSs), experiences an intra-cell interference (Ior) to inter-cell interference (Ioc) ratio (Ior/Ioc) of 0 dB. The intra-cell interference (Ior) is the total received signal power in a time slot from the BS in which the WTRU is communicating. The inter-cell interference (Ioc) is the sum of the total received signal power in the same TS from all the neighboring BSs. Intra-cell interference (Ior) is therefore the “useful” energy, or the signal from the BS with which the WTRU is communicating. Inter-cell interference is the interference caused by all the undesired signal energy from all the other BSs received by the WTRU, and is therefore detrimental to the decoding of the “useful” signal.
This Ior/Ioc ratio is an important parameter for the performance of a multiuser detector (MUD). The analogous ratio which is found in the more classic signal detectors, such as RAKE receivers in frequency division Duplex (FDD) is Ec/Io, where “Ec” is the energy per chip of the desired spreading code and Io is the sum of the energies of all other spreading codes which the WTRU can receive, but does not need to decode. As the geometric path-loss situation is extended to more than just the two closest neighbors, the Ior/Ioc ratio will continue to decrease and approach −1.5 dB.
Shadowing and fading will make the communications worse and more sporadic. In fading environments, it is anticipated that an Ior/Ioc in the range of at least −1 to 0 dB or higher is needed in order to decode the BCH properly with reasonable acquisition time. Analysis has shown that for a time-aligned BCH TS communication system, approximately 25% of WTRUs at cell borders and 15% within all cell areas will experience an Ior/Ioc<−1 dB. This results in a very degraded BCH beacon detection which leads to detrimental effects on the user's perception of quality-of-service. As the Ior/Ioc value decreases, the BCH beacon acquisition is compromised and WTRU synchronization under the worst-case circumstances would be impossible for a significant part of the deployment area.
It is therefore desirable to provide a novel beacon TS utilization approach to obviate the disadvantages discussed above.